Color filters are generally produced by forming a black matrix (frame) on the surface of a substrate made of a transparent material, such as glass or a sheet-type resin, and sequentially forming mosaic pixels (color patterns) using three or more different colors, including red (R), green (G) and blue (B) colors. Representative methods for producing color filters are dyeing, printing, pigment dispersion, electrodeposition, and other methods.
Of these, pigment dispersion methods are mainly employed to produce color filters. According to pigment dispersion methods, color filters are produced by applying a photosensitive composition comprising a pigment to a transparent substrate, exposing the coated substrate to light through a patterned photomask, developing the exposed substrate using an aqueous alkaline solution as a developer, and curing (drying) the developed substrate at a high temperature. Pigment dispersion methods are known to have advantages of high precision in the position of pixels on the color filters and film thickness, superior physical properties, including superior stability against light, heat and chemicals, and few defects (e.g., pinholes).
Generally, a black matrix is formed in a lattice, linear or mosaic arrangement between R, G and B pixels formed on a color filter, and plays a role in inhibiting mixing of colors to achieve improved contrast and in preventing the malfunction of a thin-film transistor (TFT) caused due to leakage of light emitted from backlight units. Accordingly, black matrices are required to have good light-shielding properties. The degree of light-shielding properties of black matrices is expressed as an optical density (OD) value. The optical density value is expressed as an absolute value of the common logarithm of a transmittance measured at a particular wavelength or a particular wavelength range, which indicates that the higher the OD value, the better the light-shielding properties of a black matrix.
Conventional black matrices are formed by forming a metal or a metal oxide, such as chromium or chromium oxide, into a thin film. Specifically, conventional black matrices are formed by depositing a metal, e.g., chromium, on a transparent substrate, treating the deposited substrate by photolithography, and etching the metal layer. Although the conventional black matrices thus formed exhibit superior light-shielding properties because of their small film thickness and high precision, they have drawbacks that the formation procedure is complicated and dangerous, high costs are involved due to low productivity, and environmental problems are caused by waste solutions generated during etching.
Under these circumstances, methods for forming non-toxic resin-based black matrices have recently been introduced. According to these methods, a light-shielding material and an organic pigment, such as non-toxic carbon black, are dispersed in a metal or metal oxide (e.g., chromium or chromium oxide) to prepare a photosensitive resin, and using the photosensitive resin in the formation of black matrices.
Conventional methods for forming black matrices by using carbon black are disclosed in Japanese Patent Laid-open No. 2004-292672, U.S. Pat. No. 4,762,752, Japanese Patent Laid-open No. Hei 10-204321, Japanese Patent Laid-open No. 2004-251946, Japanese Patent Laid-open No. 2004-29745, Japanese Patent Laid-open No. 2004-4762, Japanese Patent Laid-open No. 2004-75985, Japanese Patent Laid-open No. 2004-198717, Korean Patent No. 2002-0075502, Japanese Patent Laid-open No. Hei 11-60989, Japanese Patent Laid-open No. Hei 10-253820, Japanese Patent Laid-open No. Hei 10-10311, Japanese Patent Laid-open No. Hei 9-22653, etc. Most of these conventional methods are associated with primary particles of carbon black, kinds according to the structures of the primary particles of carbon black, and the surface modification of carbon black and the kind of surface-modifying agents for imparting superior resistance and dispersibility to the carbon black. Although the characteristics of carbon black dispersions, including stability, light-shielding properties, adhesive properties and resistance, are improved by conventional methods, no mention is made regarding the uniformity and resolution of black matrix patterns. Japanese Patent Laid-open No. 2004-198717, which was filed by Showa Denko K K, Japan, teaches an improvement in the resolution of black matrix patterns, but few studies have been done to satisfy the level required by liquid crystal display manufacturers.
As liquid crystal screens become larger from those of mobile devices (e.g., cell phones and notebooks) and computer monitors to those of television (TV) sets, the brightness of the liquid crystal screens has been an important factor determining the quality of liquid crystal displays. Taking into consideration the fact that high fineness of black matrices for color filters contributes to an improvement in the brightness of liquid crystal screens, the uniformity and resolution of black matrix patterns are gaining importance.